The performance of an antenna is determined by several parameters, one of which is efficiency. For a microstrip antenna, "efficiency" is defined as the power radiated divided by the power received by the input to the antenna. A one-hundred percent efficient antenna has zero power loss between the received power input and the radiated power output. In the design and construction of microstrip antennas it is desirable to produce antennas having a relatively high efficiency rating, preferably in the range of 95 to 99 percent.
One factor in constructing a high efficiency microstrip antenna is minimizing power loss, which may be caused by several factors including dielectric loss. Dielectric loss is due to the imperfect behavior of bound charges, and exists whenever a dielectric material is located in a time varying electrical field. Moreover, because dielectric loss increases with operating frequency, the problem of dielectric loss is aggravated when operating at higher frequencies.
The extent of dielectric loss for a particular microstrip antenna is determined by, inter alia, the permittivity, .di-elect cons., expressed in units of farads/meter (F/m), of the dielectric space between the radiator and the ground plane which varies somewhat with the operating frequency of the antenna system. As a more convenient alternative to permittivity, the relative dielectric constant, .di-elect cons..sub.r, of the dielectric space may be used. The relative dielectric constant is defined by the equation:
.di-elect cons..sub.r =.di-elect cons./.di-elect cons..sub.o
where .di-elect cons.is the permittivity of the dielectric space and .di-elect cons..sub.o is the permittivity of free space (8.854.times.10.sup.-12 F/m). It is apparent from this equation that free space, or air for most purposes, has a relative dielectric constant approximately equal to unity.
A dielectric material having a relative dielectric constant close to one is considered a "good" dielectric material--that is, the dielectric material exhibits low dielectric loss at the operating frequency of interest. When a dielectric material having a relative dielectric constant equal to unity is used, dielectric loss is effectively eliminated. Therefore, one method for maintaining high efficiency in a microstrip antenna system involves the use of a material having a low relative dielectric constant in the dielectric space between the radiator patch and the ground plane.
Furthermore, the use of a material with a lower relative dielectric constant permits the use of wider transmission lines that, in turn, reduce conductor losses and further improve the efficiency of the microstrip antenna.
The use of a material with a low dielectric constant, however, is not without drawbacks. One typical drawback is that it is difficult to produce high-speed compact patch antennas spaced from a ground plane by a "good" dielectric. When a dielectric material disposed between a patch and a ground plane has a low dielectric constant (about 1), the resulting patch size is large (for example at 3.6 GHz patches of about 1550 mm.sup.2 result). For mobile applications and for use in arrays, such a patch size is often problematic.
Another problem with antennas as described above is that the feed efficiency often degrades substantially as the patch is spaced further away from the ground plane. That said, more spacing of the patch from the ground plane is often advantageous and, as such, is usually accommodated using dielectric material with a higher dielectric constant to fill the space between the patch and the ground plane. Unfortunately, efficiency is substantially compromised in order to meet other design parameters.
It would be advantageous to provide a patch antenna that is spaced a distance from a ground plane and efficiently coupled to a feed absent a substrate having a high dielectric constant filling the space therebetween.